U.S. patent number 4,814,407 [Application Number 06/944,830] was granted by the patent office on 1989-03-21 for composition for waterproofing silica-ceramic insulation bodies.
This patent grant is currently assigned to PCR, Inc.. Invention is credited to Levy A. Canova, Ralph J. DePasquale, Michael E. Wilson.
United States Patent |
4,814,407 |
Canova , et al. |
March 21, 1989 |
Composition for waterproofing silica-ceramic insulation bodies
Abstract
There is provided a method and compositions for improving the
resistance to absorption of water by a porous rigid sintered and
pressed block of short staple amorphous silica fiber by
distributing within the pores of said block an alkylalkoxysilane or
a fluoroalkylalkoxysilane which is substantially free of
deleterious halide. Halide may be reduced by special purification
techniques or by netralization in situ with acid scavanger.
Inventors: |
Canova; Levy A. (Orange Park,
FL), DePasquale; Ralph J. (Jacksonville, FL), Wilson;
Michael E. (Jacksonville, FL) |
Assignee: |
PCR, Inc. (Gainesville,
FL)
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Family
ID: |
27112335 |
Appl.
No.: |
06/944,830 |
Filed: |
December 22, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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731978 |
May 8, 1985 |
4649063 |
Mar 10, 1987 |
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Current U.S.
Class: |
528/21; 528/28;
528/31 |
Current CPC
Class: |
C04B
41/4922 (20130101); C04B 41/4933 (20130101) |
Current International
Class: |
C04B
41/45 (20060101); C04B 41/49 (20060101); C08G
077/06 () |
Field of
Search: |
;528/28,21,31 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0500159 |
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Feb 1954 |
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CA |
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59-066422 |
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Apr 1984 |
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JP |
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Primary Examiner: Page; Thurman K.
Attorney, Agent or Firm: Hedman, Gibson, Costigan &
Hoare
Parent Case Text
RELATED APPLICATION
The application is a continuation in part of our copending
application Ser. No. 731,978 filed May 8, 1985, now U.S. Pat. No.
4,649,063, dated Mar. 10, 1987.
Claims
What is claimed is:
1. A composition of matter comsisting essentially of (a) a silane
having the general formula:
wherein R is an alkyl group or fluorine substituted alkyl group
containing from 1 to 3 carbon atoms, each occurrence of R' is
independently selected from hydrogen, or aryl, or alkyl, or
alkylene, or fluorine substituted alkyl or alkylene group
containing from 2 to 10 carbon atoms, R" is an alkyl group or
fluorine substituted alkyl group containing from 2 to 4 carbon
atoms, Q is oxygen, nitrogen, or sulfur, n is 1 or 2, y is 0 or 1,
and when y is 1, R' is alkylene or fluorine substituted alkylene
containing from 2 to 10 carbon atoms and (b) up to 5.0% by weight
of an acid scavenging agent having a boiling point at atmospheric
pressure of up to about 200.degree. C.
2. A composition as defined in claim 1 wherein the silane is
isobutyltrimethoxysilane.
3. A composition as defined in claim 1 wherein the acid scavenging
agent is a trialkylamine.
4. A composition as defined in claim 1 wherein the acid scavenging
agent is triethylamine.
5. A composition as defined in claim 1 wherein the acid scavenging
agent is tetramethyldisilazane.
6. A composition of matter consisting of about 98% by weight
isobutyltrimethoxysilane and about 2% by weight of
triethylamine.
7. A composition as defined in claim 1 wherein the silane is
isobutyltriethoxysilane.
8. A composition as defined in claim 1 wherein the silane is
dimethylethoxysilane.
9. A composition as defined in claim 1 wherein said acid scavenging
agent is selected from ethylene diamine, dipropylamine,
isopropanolamine, amylamine, monoethanolamine, aniline,
methylaminobenzene, hexylamine, cyclohexylamine, octylamine,
2-ethyl-1-aminohexane, triethylamine, tetramethyldisilazane,
ammonia, diethylaminotrimethylsilane, propylene oxide, alloocimene,
alpha-methylstyrene, isobutylene, methylisocyanate or a mixture of
any of the foregoing.
Description
This invention relates, as indicated, to a composition for
waterproofing siliceous bodies, and more particularly to a
composition for improving the resistance to absorption of water by
a rigid, porous sintered and pressed block of short staple
amorphous silica fiber, especially such blocks as are used or
reusable space vehicles, e.g., a shuttle orbiter.
BACKGROUND OF THE INVENTION AND PRIOR ART
By now, knowledge of the space shuttle and its abilities to exit
from the earth's atmosphere, enter an orbit of elective duration,
perform various functions in space, re-enter the earth's
atmosphere, land safely and be reconditioned for succeeding flights
into space is widely known. It is also fairly generally known that
the outer surface of the space shuttle is covered with heat
resistant "tiles", and that from time to time difficulty has been
experienced in various of these tiles coming loose from the shuttle
surface and being lost. Beyond the foregoing and the media
presentations of weightlessness and the spectacular takeoff and
landing episodes, very little else is generally known.
In an article entitled "The Toughest Title Job Ever" by Robert L.
Dotts, Donald M. Curry and Donald J. Tillian, printed in Chemtech
for October 1984, pages 616-626, there is presented a very well
written description of the nature and properties of the "Unique
thermal protection system (TPS)" that enshrouds the entire outer
skin of the space shuttle. As pointed out in the article, the
thermal protection system is designed to function for 100 missions
with minimal weight gain, maintenance, and refurbishment. The
thermal protection system must operate successfully in a variety of
environments. The system must be capable of maintaining during
ascent and re-entry temperatures of the orbiter's structures below
350.degree. F. The thermal protection system must also withstand
launch acoustics, structural deflections and temperatures
encountered in deep space as well as earth environments including
salt spray, fog, wind, and rain. Different thermal protection
materials are applied to different areas of the outer surface of
the orbiter depending upon the conditions encountered in the
various environments mentioned above. Three of the material systems
used to protect the structure of the orbiter are reusable. Two of
these systems are formed of a low density silica-ceramic insulation
and the third material is a coated nylon felt system. The
silica-ceramic tiles are classified as high temperature and low
temperature reusable surface insulation, the primary difference
between these two being in the nature of the surface coating. The
high temperature tiles are coated with a black borosilicate glass,
whereas the low temperature tiles are coated with a white
borosilicate glass.
The basic silica-ceramic insulation material is manufactured in two
densities, one at nine pounds per cubic foot and the other at
twenty-two pounds per cubic foot. These materials together cover
70% of the orbiter structure. Most of the area is covered with the
lower density material whereas the higher density material is used
in areas where a more durable material is necessary. The
silica-ceramic material is formed from a short-staple 99.6% pure
amorphous silica fibers which are slurried, felted, pressed and
sintered to form rigid blocks of insulation material. The
individual tiles are then cut from the blocks of insulation
material to predetermine size. Ordinarily, the blocks are
approximately 6 inches by 6 inches by 1 inch with outer surfaces
which are planar. Geometry of the vehicle determines, of course,
the shape of other tiles.
To provide a barrier to moisture absorption, the tiles are coated
with a borosilicate glass. Those tiles which are adapted to high
tempeature surfaces have a coating that contains a black pigment
(silicontetraboride).
After coating, the tiles are rendered water repellent to prevent
water absorption into the low density insulation. Thereafter the
tile remains water repellant until exposed to the high temperatures
of reentry.
The tiles are bonded to the outer surface of the orbiter using a
polysilicone adhesive and an intervening layer of nylon felt.
Although the silica-ceramic tile is a highly effective insulator,
it is nevertheless ceramic and possesses low mechanical strength.
To prevent damage to the tiles by flexure of the airframe and
consequent induction of stresses in the tile, the nylon felt
material is used to isolate such strains and prevent damage to the
individual tiles. Gaps between the individual tiles are filled with
a suitable spacer material.
As pointed out in the aforesaid article, a major technical problem
encountered in the flight testing program has been keeping moisture
out of the tile. Further details of the moisture absorption
characteristics of the orbiter's thermal protection system are
founded in the article by Schomburg, Dotts, and Tillian entitled
"Moisture Absorption Characteristics of the Orbiter Thermal
Protection System and Methods Used to Prevent Water Ingestion",
Intersociety Conference on Environmental Systems, July 11-15,
1983.
The present invention has for a primary object the enhancement of
the resistance to absorption of water by the silica-ceramic tiles.
It has been found that organic silane compositions which have been
modified to remove or neutralize substantially completely
deleterious impurities in said compositions or the tile to be
waterproofed are especially effective to the accomplishment of the
foregoing ends.
BRIEF STATEMENT OF THE INVENTION
Briefly stated, the present invention is in a composition for
improving the resistance to absorption of water by a porous rigid
sintered and pressed body of short staple amorphous silica fiber
which comprises an alkylalkoxysilane substantially free of
deleterious halide and having the following general formula:
wherein R is an alkyl group of fluorine substituted alkyl group
containing from 1 to 3 carbon atoms, R' at each occurrence thereof
is independently selected from hydrogen, or aryl, or alkyl, or
alkylene, or fluorine substituted alkyl or alkylene group
containing from 2 to 10 carbon atoms, R" is an alkyl group or
fluorine substituted alkyl group containing from 2 to 4 carbon
atoms, n is 1 or 2, y is 0 or 1, and when y is 1, R' is alklene or
fluorine substituted alklene containing from 2 to 10 carbon atoms.
A heteroatom, Q, may be present in the alkyl substituent attached
to silicon such as O, N, or S.
DETAILED DESCRIPTION OF THE INVENTION
The use of alkoxysilanes as agents to decrease the absorption of
water through porous bodies, particularly ceramic bodies (U.S. Pat.
No. 2,774,690, U.S. Pat. No. 2,893,898 and numerous others) is well
known. The silanes of commerce today are usually produced by using
as a principal reactant, trichlorosilane. This material can be
reacted directly with alcohol to produce a trialkoxysilanehydride,
or with an unsaturated hydrocarbon to attach an alkyl group
directly to silicon through a silicon-carbon bond. Thereafter, the
alkyltrichlorosilane may be reacted with a material such as methyl
alcohol or ethyl alcohol, for example, to form the corresponding
alkoxysilanes. Various modifications of these procedures are well
known.
It has been found that residual chloride remaining after the
preparation of commercial examples of these silanes is detrimental
to the waterproofing efficacy of the silane in the tiles. It has
also been found that heavy metals such as titanium which may be
used in the formation of the silane at one or another stage also
interfere adversely with the performance of the titles. Commercial
preparations of various silanes may contain impurities of the type
described, particularly chloride, to the extent of more than 1,000
ppm of chloride. Unless the content of these impurities is reduced
below about 50 ppm, and preferably less than 20 ppm, the
performance of the tiles tends to be unsatisfactory. For example,
with isobutyltrimethoxysilane containing less than 15 ppm chloride
exceptionally satisfactory results have been obtained when using
this material as a moisture resistance improving agent.
A material which has been used in the past in waterproofing
silica-ceramic tiles for an orbiter device is hexamethyldisilazane
(HMDS). This material when injected into the center of the tile and
distributed throughout the tile body provides good water
resistance. However, upon decomposition, the material yields
ammonia which apparently has an advantage effect and causes
reversion of the siloxane polymer adhesive. The balance of the
molecule acts as an end-capper and prevents reformation of the
polymer which would otherwise occur. The reaction thus causes an
irreversible softening of the adhesive which can be related to
adhesive failure. While the alkalkoxysilane does not yield such a
deleterious substance, the presence of chloride and the presence of
a heavy metal such as titanium were found to be deleterious.
In general, the silanes used as waterproofing agents should have a
boiling point above about 50.degree. C. and below about 250.degree.
C., and preferably in the range from 100.degree. C. to 160.degree.
C. at atmospheric pressure. Thus, these are normally liquid
materials and are readily amenable to the present method of
effecting waterproofing of the silica-ceramic tiles. This is
accomplished by injecting through a hypodermic needle from 0.5 to 6
ml of the waterproofing agent into the center of the tile. By
maintaining the tile at ambient temperature, the silane diffuses to
all parts of the tile. Usually 24 hours is sufficient but the time
may be as long as 72 to 96 hours. Other methods of introducing the
silane are possible, e.g., maintaining a vapor of the silane in
contact with the tiles.
It becomes convenient at this point to give specific examples of
silanes useful in carrying out this invention. These are for
illustrative purposes only, and those skilled in the art are aware
of the desired properties given above will be able to suggest other
silanes of equal utility for use herein.
Ethyltriethoxysilane
Propyltriethoxysilane
Propylmethyldiethoxysilane
Propyldiethylethoxysilane
n-butyltrimethoxysilane
Isobutyltrimethoxysilane (best known material)
Isobutyltriethoxysilane
Isobutyldiethoxysilane
Cyclohexyltrimethoxysilane
Cyclobutyltriethoxysilane
Ethoxypropyltrimethoxysilane
Methoxypropyltrimethoxysilane
Propoxyethyldimethoxymethylsilane
Diisopropyldimethoxysilane
Vinyltriethoxysilane
Vinyltrimethoxysilane
p-menthenetrimethoxysilane
Phenyltrimethoxysilane
Pentenyltrimethoxysilane
Isoamylenetrimethoxysilane
Diethylethoxysilane
Dimethylethoxysilane
Dipropylmethoxysilane
For best results the foregoing silanes should have chloride
contents less than 50 ppm, and preferably less than 20 ppm.
All of the silanes disclosed herein can be used in the tile
injecting waterproofing step in neat condition or in the presence
of a suitable low boiling (e.g., less than 150.degree. C.) solvent,
e.g., alcohol, ketone, hydrocarbon, (e.g., heptane).
Specific examples of normally liquid fluorine containing compounds
useful in the present invention include the following:
3,3,3-trifluoropropyltrimethoxysilane
3,3,3-trifluoropropylmethyldimethoxysilane
3,3,3-trifluoropropyldimethylmethoxysilane
3,3,3-trifluoropropylmethylmethoxysilane
3,3,3-trifluoropropylmethylethoxysilane
3,3,3-trifluoropropyldimethylethoxysilane
3,3,3-trifluoropropylethyldimethoxysilane
3,3,3-trifluoropropyldiethoxysilane
3,3,3-trifluoropropyldi-(2,2,1-trifluoroethoxy)silane
2-heptafluorocyclobutylethyldimethoxysilane
2-heptafluorocyclobutylethylmethyldimethoxysilane
2-heptafluorocyclobutylethyldimethylmethoxysilane
3-heptafluoroisopropoxypropyltrimethoxysilane
3-heptafluoroisopropoxypropylmethylmethoxysilane
3-heptafluoroisopropoxypropyldimethylmethoxysilane
3,3,4,4,4-pentafluorobutyltrimethoxysilane
3,3,4,4,4-pentafluorobutyldimethylmethoxysilane
3,3,4,4,4-pentafluorobutylmethylmethoxysilane
Numerous other examples will become clear to those skilled in the
art from the foregoing examples. As a further guide, the useful
fluoroalkylalkoxysilanes have the following general formula:
##STR1## wherein R' is selected from CF.sub.3, C.sub.2 F.sub.5,
cycloC.sub.4 F.sub.7, and (CF.sub.3).sub.2 CFO-CH.sub.2 ; R.sup.2
is selected from hydrogen and C.sub.1 -C.sub.3 alkane; R.sup.3 is
selected from hydrogen and C.sub.1 -C.sub.3 alkane, R.sup.4 is
selected from 0-(C.sub.1 -C.sub.3) alkyl, and OCH.sub.3 CF.sub.3 ;
m is 0, 1, or 2; n is 0, 1, or 2; and m+n is 0, 1, or 2. These
materials may be used to waterproof silica-ceramic tiles in exactly
the same manner as the preferred alkyalkoxysilanes.
The active waterproofing agents of the present invention are
conveniently prepared from a halosilane containing one to three
chlorine or other halogen atoms directly connected to silicon,
e.g., HSiCl.sub.3. Esterification with alcohol such as methyl
alcohol, ethyl alcohol or isopropyl alcohol to introduce one or
more alkoxy groups usually leaves residual chloride in the product.
While the amount of such chloride is very small less than about
0.3% by weight, such amounts as are normally present have been
found to be very deleterious to the coatings, especially the
polysiloxane adhesives on the tiles. This can be responsible for
loss of adhesion under the environments encountered by the orbiter.
Loss of tiles is then encountered which can be very damaging on
reentry and dangerous to the occupants.
There are two principal ways we have found by which the deleterious
effect of chloride in the waterproofing agents hereof can be
counteracted. In the first case, we have found that if we
chemically remove adventitious halide, e.g., chloride, by treatment
with an alkali metal alkoxide, e.g., sodium methoxide, the halide
content, which normally ranges about 500 ppm to as high as 3000 ppm
or more can be reduced to less than 50 ppm, and preferably less
than 20 ppm. When the silane waterproofing agent contains no more
than 50 ppm and preferably no more than 20 ppm halide, excellent
results are obtained with the tiles in terms of resistance to
absorption of water.
Another method which has been, found for nullifying the deleterious
effects of halide in the silane waterproofing agents hereof is to
chemically tie up advantageous halide by blending into the silane
prior to injection into the tile up to 5% by weight of the silane,
and preferably from 1% to 3% by weight, of an organic base, usually
an organic amine. 2% by weight of triethylamine, for example, gives
excellent results.
The following specific examples illustrates a simple and preferred
procedure for decreasing halide content of silane.
EXAMPLE I
An initial attempt to remove chlorides from commerical
isobutyltrimethoxysilane, Prosil 178, a product of SCM Specialty
Products, by fractionation was unsuccessful. Two fractionations at
11:1 reflux ratio reduced the chloride level to a minimum of 237
ppm. One of the fractions containing 339 ppm chloride was stirred
at 50.degree. C. for four hours with 3.7% by weight of sodium
methoxide. The reacted material was strip distilled at atmospheric
pressure to give a product with 13.1 ppm chloride. This procedure
was repeated with a larger sample. This time the material was
stirred at 100.degree. C. for three hours with 2.3% sodium
methoxide before strip distilling. The heat cut of the
isobutyltrimethoxysilane contained 14.1 ppm chloride. This sample
was used to waterproof a test tile described below.
It was attempted to remove chlorides from a larger amount of Prosil
178 without doing a preliminary fractionation. Treatment with
sodium methoxide for two hours followed by strip distillation gave
a product with 302 ppm chloride.
EXAMPLE II
A batch of Prosil 178 was fractionated and a 140 gm fraction
treated with 3 gm sodium methoxide at 100.degree. C. for two hours.
The chloride level in the distilled product was 19 to 26 ppm
(initially 160).
EXAMPLE III
It was found that addition of methanol to the mixture of sodium
methoxide and Prosil 178 produced a very low chloride content. A
438 gm sample of distilled Prosil 178 (chloride=612 ppm) was
stirred three hours at 100.degree. C. with 4.3 gm sodium methoxide
and 2.2 gm anhydrous methanol. Strip distillation gave material
with 8 ppm chloride.
A 3,058 gm charge of Prosil 178 was fractionated after heating with
31 gm sodium methoxide and 15 gm anhydrous methanol. Cuts 5-8 were
blended to produce 1,446 gm that contained 13.7 ppm chloride. This
process combines fractination and treatment with sodium methoxide.
This is the best mode known to us and we would use it for
commercial production of low chloride Prosil 178
(isobutyltrimethoxysilane). Other chloride sequestering agents,
e.g., silver nitrate, sodium acetate, sodium metal, sodium
carbonate, sodium/potassium alloy, etc., are well known and may be
used with equal effect. Organic agents may also be used, e.g.,
organic epoxides.
The product of Example I containing 14.1 ppm Cl was tested as
follows on virgin silica-ceramic space shuttle tiles
6".times.6".times.1".
Virgin tiles coated with borosilicate glass were used in testing
silane candidates. It was desired to compare the waterproofing
efficiency of Prosil 178 having a typical chloride level (1,000
ppm) with a highly purified, low chloride, Prosil 178. Tiles were
injected in the center of the 6".times.6" face to a depth of 0.25
to 0.5 inch, with 1 ml of each sample. To our surprise, the
low-chloride material was a much better waterproofing agent than
the typical high chloride material. Waterproofing efficiency was
determined by immersion of tiles in an aqueous solution of
methylene blue for five minutes and weighing to determine the
weight increase.
______________________________________ PROSIL: SAMPLE CODE PPM Cl %
WT. INCREASE ______________________________________ Commerical
Prosil 178 1483 4.3 to 5.0 Example I 14.1 1.0 to 1.3
______________________________________
The tile treated with Example I was completely waterproofed except
for two corners. The high chloride Prosil 178 left a blue area
about 13/4" in diameter around the point of injection and was
unacceptable.
Specific illustrative examples of acid scavangers useful herein are
as follows:
ethylene diamine
dipropylamine
isopropanolamine
amylamine
monoethanolamine
aniline
methylaminobenzene
hexylamine
cyclohexylamine
octylamine
2-ethyl-1-aminohexane
triethylamine (best known example)
tetramethyldisilazane
ammonia
diethylaminotrimethylsilane
propylene oxide
alloocimene
alpha-methylstyrene
isobutylene
methylisocyanate.
A specific example of an exceptionally successful waterproofing
agent is 98 low chloride isobutyltrimethoxysilane and 2%
triethylamine. Another exceptional example is 98
isobutyltrimethoxysilane and 2% tetramethyldisilazane. Still
another example is 97.5% triflurorpropyltrimethoxysilane and 2.5%
triethylamine. Another example is 98% high chloride
isobutyltrimethoxysilane and 2% triethylamine.
It should be noted that mixtures of the foregoing silanes may be
used as waterproofing agents with or without added acid
scavengers.
While the previous disclosure has been directed primarily to alkoxy
grous as the hydrolyzable groups, other such groups may replace
part or all of the alkoxy groups. For example, there may be present
acetate, oximino, thioalkoxy, trifluoroacetate, dialkylamino,
etc.
By "substantially free of deleterious halide" as used herein and in
the appended claims is meant that the chloride has either been
removed as described above, or rendered innocuous in the system by
the addition of a small amount of an acid scavenging agent or a
combination of both.
* * * * *